1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Modern wireless RF transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes such as frequency shift keying (FSK) and phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. Independent of the particular communications scheme employed, the RF transmitter output signal, sRF(t), can be represented mathematically assRF(t)=r(t)cos(2πfct+θ(t))  (1)where fc denotes the RF carrier frequency, and the signal components r(t) and θ(t) are referred to as the envelope and phase of sRF(t), respectively.
Some of the above mentioned communication schemes have constant envelope, i,e.,r(t)=R, and these are thus referred to as constant-envelope communications schemes. In these communications schemes, θ(t) constitutes all of the information bearing part of the transmitted signal. Other communications schemes have envelopes that vary with time and these are thus referred to as variable-envelope communications schemes. In these communications schemes, both r(t) and θ(t) constitute information bearing parts of the transmitted signal.
The most widespread standard in cellular wireless communications is currently the Global System for Mobile Communications (GSM). A second generation GSM standard employs Gaussian Minimum Shift Keying (GMSK), which is a constant-envelope binary modulation scheme allowing raw transmission at a maximum rate of 270.83 kilobits per second (kbps). In any mobile communication system, the radio spectrum is a very limited resource shared by all users. GSM employs a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) for the purpose of sharing the spectrum resource. GSM networks typically operate in the 900 MHz frequency range. The radio spectrum in the bands 890-915 MHz is for the uplink (mobile station to base station) and 935-960 MHz is for the downlink (base station to mobile station). The spectrum for both uplink and downlink is divided into 200 kHz wide carrier frequencies using FDMA, and each base station is assigned one or more carrier frequencies. Each carrier is divided into eight time slots using TDMA. Eight consecutive time slots form one TDMA frame, with a duration of 4.615 ms. A physical channel occupies one time slot within a TDMA frame. Each time slot within a frame is also referred to as a burst. TDMA frames of a particular carrier frequency are numbered, and formed in groups of 26 or 51 TDMA frames called multi-frames. While GSM is sufficient for standard voice services, future high-fidelity audio and data services demand higher data throughput rates.
General Packet Radio Service (GPRS) is a new non-voice service that allows information to be sent and received across a mobile telephone network. It supplements today's Circuit Switched Data (CSD) and Short Message Service (SMS). GPRS employs the same modulation scheme as GSM, but higher data throughput rates are achievable with GPRS since it allows for all eight timeslots to be used by a mobile station at the same time.
Even higher data rates are achieved in the specification of the Enhanced Data rates for GSM Evolution (EDGE) cellular telephony standard by selectively applying a 3π/8 offset, 8-level PSK (8-PSK) modulation scheme. With this variable-envelope communication scheme, the maximum bit rate is tripled compared to GSM, while the chosen pulse shaping ensures that the RF carrier bandwidth is the same as that of GSM, allowing for the reuse of the GSM signal bandwidths. Additionally, to further increase the flexibility of data transmission, so-called multi-slot operation has been introduced into GSM/GPRS/EDGE systems. In multi-slot operation, more than one time slot out of the eight in one GSM frame can be used for transmission with GMSK and/or 8-PSK modulation.
As mentioned above, the GMSK modulation scheme of standard GSM is an example of a constant envelope communications scheme. An example transmitter appropriate for such constant-envelope modulation schemes in a mobile station unit is a translational loop transmitter. In this transmitter, the digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF. The resulting digital signal is converted to analog using a digital-to-analog converter (DAC) and a low pass filter (LPF) that filters out undesired digital images of the IF signal. A translational loop, essentially a phase locked loop (PLL), then translates, or up-converts, the IF signal to the desired RF signal and a power amplifier (PA) delivers the appropriate transmit power to the antenna.
As mentioned above, the 8-PSK modulation scheme of EDGE is an example of a variable envelope communications scheme. In practice, the power spectrum emitted from an EDGE transmitter will not be ideal due to various imperfections in the RF transmitter circuitry. Thus, quality measures of the transmitter performance have been established as part of the EDGE standard and minimum requirements have been set. One quality measure that relates to the RF signal power spectrum is the so-called spectral mask. This mask represents the maximum allowable levels of the power spectrum as a function of frequency offset from the RF carrier in order for a given transmitter to qualify for EDGE certification. In other words, the spectral mask requirements limit the amount of transmitter signal leakage into other users' signal spectrum. For example, at a frequency offset of 400 kHz (0.4 MHz), the maximum allowable emission level is −54 dB relative to the carrier (dBc). Another RF transmitter quality measure of the EDGE standard is the modulation accuracy, which relates the RF transmitter modulation performance to an ideal reference signal. Modulation accuracy is related to the so-called error vector magnitude (EVM), which is the magnitude of the difference between the actual transmitter output and the ideal reference signal. The error vector is, in general, a complex quantity and hence can be viewed as a vector in the complex plane. Modulation accuracy is stated in root-mean-square (RMS), 95th percentile, and peak values of the EVM and is specified as a percentage. For a given transmitter to qualify for EDGE certification, the RMS EVM must be less than 9%, the 95th percentile of EVM values must be less than 15%, and the peak EVM value must be less than 30%.
The increase in system flexibility resulting from the introduction of multi-slot operation in EDGE presents the challenge of finding an efficient implementation of a joint GMSK/8-PSK modulator which enables easy and fast switching between GMSK and 8-PSK modulation in consecutive time slots. Such modulation switching must be achieved within the so-called guard interval, merely 30 microseconds (us) long. Further complication is encountered in the domain of the RF signal PA. Exploiting the fact that GMSK is a constant envelope modulation scheme, the PA can typically be driven in saturation mode for higher efficiency when transmitting GSM signals. However, due to the variable-envelope properties of the 8-PSK modulation option in EDGE, driving the PA in saturated mode is not possible. Rather, a certain power back-off of the PA input signal level is required to maintain adequate modulation accuracy. Typical transmitter powers may be 33 dBm in GMSK mode and 27 dBm in PSK mode. Thus, when switching modulation schemes in multi-slot operation from GMSK to 8-PSK, or vice versa, a change of PA input signal level must occur. Such change must be achieved within the guard interval and in such a fashion that switching transients do not violate the spectral mask requirements.
In addition, WCDMA (Wideband Code Division Multiple Access) is the world's leading third generation (3G) technology. With data rates up to 100 times those of today's networks, WCDMA will introduce a new generation of telecommunication into the world and change the way people communicate forever. Providing mobile users with data rates initially up to 384 kbps, and in later releases, up to 14 Mbps, WCDMA is an ultra high-speed, ultra high-capacity radio technology that generates and carries a new range of rich, fast, colorful media that consumers will be able to access over their mobiles (e.g., color graphics, video, digital audio, Internet and e-mail). Occupying an RF channel bandwidth of 5 MHz, WCDMA employs a “spreading sequence” with a chip rate of 3.84 million chips per second (Mcps) and a 4-level PSK, variable-envelope modulation scheme.
It is important to ensure that WCDMA, while operating in different frequency bands and larger bandwidth, is seen as an evolution of the GSM networks. This ensures that investments made in GSM networks will remain profitable for years to come while introducing the support of WCDMA service. In view of this evolution, GSM and WCDMA operators will go through three distinct phases at time progresses and market conditions change. They will move from (i) current voice-centric GSM-only businesses to (ii) nationwide low-speed GPRS wireless data services, with high-speed WCDMA services in select areas, to (iii) focusing solely on high-speed multimedia mobile internet WCDMA and GSM services. The long-term goal is to have a seamless network solution with multi-mode handsets that work on both GSM and WCDMA frequencies, and a network that combines the GSM and WCDMA resources. In the seamless network solution, services are provided over GSM or WCDMA radio access, depending upon radio source availability and service demand, without any input or knowledge of users.
For example, a user boarding a train in a large city and heading for the countryside can carry a multi-mode GSM/WCDMA handset, and maintain a subscription with an operator that has a GSM network with nationwide EDGE coverage. In addition, the operator also has a WCDMA network with coverage in all major cities. If the user is initially is in an area with WCDMA coverage, the user starts a combination voice and video call in the WCDMA mode. As the train moves out of the WCDMA coverage area, the network moves the call to the GSM network, renegotiates the data transfer with the handset application and uses EDGE functionality in the GSM network to continue to send both video and voice. The user now experiences the same voice quality as before, but lower quality for the streaming video.
As mentioned previously, it is important to ensure that WCDMA is seen as an evolution of the GSM networks. As a result, for many years to come, so-called multi-mode radios capable of operating in both GSM and WCDMA signal bandwidths will be in demand. In addition, as mentioned previously, it is important to meet mask requirements when modulation switching occurs (e.g., between GMSK and PSK). Thus, there is a need for a multi-mode radio transceiver capable of operating in multiple RF bands. In addition, there is a need for a multi-mode radio transceiver that is capable of switching between modulation modes, while adhering to spectral mask requirements.